Both above methods work, I would add that using UV-vis data can be tricky because many different equations to extract Eg from the data exist that may or may not be sensitive to defect absorption and scattering at low energies. Also, unless the plots are very linear, the fits to find the x-intercept are not that accurate, i.e. depending on where exactly you fit the data Eg can vary up to 0.5eV.

I think the most reliable can be photoluminescence. It requires sufficient yield and the knowledge that it is mainly band-to-band transitions that luminesce, but if that is clear the analysis is simple - the peak position is the band gap. It also gives you a convenient method to monitor recombination which I assume you're interested in since CZTS is usually researched for solar cells.

The most simple method - particularly for a thin film - would be to collect the UV-Vis-NIR absorbtion with a diffuse reflectance method. You can then plot the absorbtion versus energy. You then fit a straight line to the 'edge' of the UV-Vis spectra and the x intercept is the band gap. You can see this done in this publication in figure 9.

http://pubs.rsc.org/en/content/articlehtml/2013/ra/c2ra21684a

you can also find a description here:

http://www.perkinelmer.com/Content/applicationnotes/app_uvvisnirmeasurebandgapenergyvalue.pdf

As mentioned above, the optical absorption or transmittance study can be used to determine the bandgap. Optical techniques are very accurate. If you go one step ahead and obtain the Tauc plot then you can also determine whether the material has a direct bandgap or indirect bandgap.  Alternatively, for semiconductors, the bandgap can also be determined electrically by studying the variation of resistivity with temperature in extrinsic conduction region.

Both above methods work, I would add that using UV-vis data can be tricky because many different equations to extract Eg from the data exist that may or may not be sensitive to defect absorption and scattering at low energies. Also, unless the plots are very linear, the fits to find the x-intercept are not that accurate, i.e. depending on where exactly you fit the data Eg can vary up to 0.5eV.

I think the most reliable can be photoluminescence. It requires sufficient yield and the knowledge that it is mainly band-to-band transitions that luminesce, but if that is clear the analysis is simple - the peak position is the band gap. It also gives you a convenient method to monitor recombination which I assume you're interested in since CZTS is usually researched for solar cells.

Spectroscopic ellipsometry can also be used obtain dielectric properties of the material which can be used to determine the bandgap of the material of interest.

Usually,  optical absorption method can know the optical band gap.

Is there any method to estimate bandgap from XRD data?

@Beladiya: not really. Fundamentally the band gap does depend on the lattice parameter and as such you could feed the XRD results into a calculation of the band gap, but this is more work and less reliable, due to all the other variables (composition, several phases, etc).

You may find the energy band gap using absorption  and with cyclic voltametry You may even find the energy levels.

But cyclic voltametry is only suitable for solution state. For thin films, optical absorption and emission spectra are most frequently utilized to determine the bandgap.

Yes, optical spectroscopy are easy to access and easy to interpret. It gives an idea of band gap of the material. Combining with photoluminiscence  and tauc plot one can get relatively accurate band gap. Cyclicvoltametry is also known to give almost accurate band gap for redox materials. the difference between reduction and oxidation peaks evaluates the band gap in such measurements. Though CV is mostly used for solution phase samples but there are also some reports to deduce redox reactions of materials deposited on working electrode. Hope combining results of these three measurements you should have a quite reliable account of bandgap of your material.

Optical ellipsometry spectroscopy,  UV-Vis spectroscopy as well as the electrical measurement method, can be used to calculate the bandgap energy. One can find the slope of the ( lnR vs 1/T) graph, then calculate the Eg-value where: ( Eg = 2 k. slope), and k is the Boltzmann constant. 

By absorption analysis , you can determine the band gap if you know the film thickness

UV visible would be the best method. Please see the attached paper for detail.

you plot  the plot of  ( alpha h nu)(supe) versus hnu   where e dependent on direct or indirect transition semiconductor  by extrapolation , the gap is then easily determinedd

here I am attaching a paper which is best for your answer

Here you can find good explanation and band gap calculator,

http://www.instanano.com/characterization/theoretical/uv-vis-spectroscopy-band-gap-calculation/

I think the most famous method is Tauc plots

you can find some details of it on my paper:

Article Hydrothermal-assisted sol–gel synthesis of Cd-doped TiO2 nan...

Of Course 

Tauc plott

or dT/dlambda  vs  photon wavelength (lambda)

From the absorption curve (A vs E=hv or wave length lambda, A is the absorbance, ) in the visible and UV regions one can find correspond energy or the wave length at the peak of the curve and calculate the corresponding  energy which equals the energy band gap Eg. Eg = hv/kT; where h is the Planck's constant, T is the absolute temperature, k is the Universal Boltzmann constant and v is the frequency of the absorbed photon.

You can use UV visible method

Using UV or any other ranges depends on the expected wave length. If it unknown, it is advised to Scan the sample in the full range (IR, V, and UV). If the material is a semiconductor you will get the absorption; if not, it is not a semiconductor material.

I agree with Prof. Schnabel discussion to use another technique: photoluminescence and the peak position is the band gap. It also gives you a convenient method to monitor the recombination.

You can Using UV VIS.

Not an easy question... as you ask for kesterite i guess you are interested by the PV application. You'll find authors using tauc's, linear extrapolation from (I/E)QE; dQE/dλ, max PL emission, UV-vis ... Depending on what you want to discuss or not, and to what do you want to compare, one or another method will be more or less adequate. I can strongly recommand you the papers on the links below

http://www.sciencedirect.com/science/article/pii/S0927024815005280

http://aip.scitation.org/doi/abs/10.1063/1.4820250

Very interesting Question

Could We measure band-gap using STM.

The optical energy bandgap may be calculated on the basis of optical absorption by using Tauc relationship

αhν=A(hν-E)^n

Where α is the absorption coefficient, hν is the photon energy, n is the parameter connected to the distribution of the density of states, A a constant or Tauc parameter

Is this amorphous material? If so, Tauc plot could give you the distance between mobility edges. But you should be cautious of the fitting, if good linearity is not guaranteed, the extracted value may be very inaccurate. PL is not applicable to amorphous materials because of the smeared band edges and high defect densities. Even for crystalline materials, PL may only gives the excitonic energy whose difference with bandgap is usually small and depends on the carrier effective mass and semiconductor dielectric constant.

To determine the bandgap of material, the best technique is UV visible technique